High temperature cast iron with niobium and having compacted graphite structures

ABSTRACT

A product including an iron casting alloy including iron, niobium and compacted graphite structures.

This application claims the benefit of U.S. Provisional Application No.61/325,551 filed Apr. 19, 2010.

TECHNICAL FIELD

The field to which the disclosure generally relates includes cast ironalloys and structures including compacted graphite and methods of makingand using the same.

BACKGROUND

FIG. 1 is a scanning electron microscope (SEM) view illustratinggraphite in gray iron. In the production of gray iron, when thecomposition of molten iron and its cooling rate are appropriate, thecarbon (C) in the iron separates during solidification and formsseparate graphite flakes that are interconnected within each eutecticcell. The graphite grows edgewise into the liquid and forms thecharacteristic flake shape. When gray iron is broken, most of thefracture occurs along the graphite, thereby accounting for thecharacteristic gray color of the fractured surface. Because the largemajority of the iron castings produced are of gray iron, the genericterm, cast iron, is often improperly used to mean gray ironspecifically.

The properties of gray iron are influenced by the size, amount anddistribution of the graphite flakes, and by the relative hardness of thematrix metal around the graphite. These factors are controlled mainly bythe C and silicon (Si) contents of the metal and the cooling rate of thecasting. Slower cooling and higher C and Si contents tend to producemore and larger graphite flakes, a softer matrix structure and lowerstrength. The flake graphite provides gray iron with unique propertiessuch as excellent machinability at hardness levels that produce superiorwear-resisting characteristics, the ability to resist galling andexcellent vibration damping.

FIG. 2 is an SEM illustrating graphite in ductile iron. Ductile iron(also known as nodular iron) compositions typically include 3.2-4.1%carbon, 1.7-2.8% silicon, 0.45-0.8% manganese, 0.1-0.14% phosphorus,0.05-0.13% sulfur by weight. In nodular iron, the magnesium may be usedas a treatment element and is retained in the final casting in an amountof about 0.04% and sulfur is reduced to about 0.002%. However, incompacted graphite irons (described in greater detail hereafter), themagnesium is typically retained in an amount of about 0.01-0.035% or upto 0.4% by weight if titanium is added.

Castable iron-base ductile alloys can be formulated for high temperaturestrength applications. Such applications include casted hot side engineparts. Such parts include turbochargers, center housings, back plates,exhaust manifolds and integrated turbo-manifold components used in theautomobile and truck manufacturing industries. One known chemistry forsuch a castable iron-based high temperature ductile alloy includescarbon in an amount of 3.0-3.4% by weight, silicon in an amount of3.75-4.25% by weight, molybdenum in an amount of 0.5-0.7% by weight,magnesium in an amount of 0.6% by weight or less, sulfur in an amount of0.7% by weight or less, phosphorus in an amount of 0.02% by weight orless, nickel in an amount of 0.5% by weight or less, magnesium in anamount of 0.08% by weight or less, and iron and impurities.

FIG. 3 is an SEM illustrating graphite in malleable iron. Malleable ironis characterized by having the majority of its C content occur in themicrostructure as irregularly shaped nodules of graphite. This form ofgraphite is called temper carbon because it is formed in the solid stateduring heat treatment. The iron is cast as a white iron of a suitablechemical composition. After the castings are removed from the mold, theyare given an extended heat treatment starting at a temperature above1650° F. (900° C.). This causes the iron carbide to dissociate and thefree carbon precipitates in the solid iron as graphite. The rapidsolidification rate that is necessary to form the white iron limits themetal thickness in the casting that is practical for the malleable ironprocess.

A wide range of mechanical properties can be obtained in malleable ironby controlling the matrix structure around the graphite. Pearlitic andmartensitic matrices are obtained both by rapid cooling through thecritical temperature and with alloy additions. Malleable ironscontaining some combined carbon in the matrix often are referred to aspearlitic malleable, although the microstructure may be martensitic or aspheroidized pearlite.

FIG. 4 is an SEM illustrating graphite in compacted graphite iron. Incompacted graphite iron, the graphite occurs as blunt flakes that areinterconnected with each cell. This compacted graphite structure andresulting properties of the iron are intermediate between gray andductile irons. The compacted graphite shape is also called quasiflake,aggregated flake, seminodular and vermicular graphite (vermiculite). Aswill be appreciated from FIGS. 1, 2 and 4, compacted graphite iron has astructure intermediate between those of gray flake iron and nodularductile iron. Known methods of making compacted graphite iron includethe Bruhl Oxycast route or the SinterCast method. Compact graphite ironwas developed primarily to provide an iron that did not need extensivealloying but would be stronger than gray iron while being easier tomachine than nodular iron. Compacted graphite iron typically has a 35%greater stiffness and a 75% higher tensile strength than gray iron and ahigher fatigue strength than aluminum at automotive engine operatingtemperatures. Tensile strengths between 250 and 450 MPa, arecommercially available (yield strengths 175 to 315 MPa). One known usefor such compacted graphite iron alloys is in the production of cylinderblocks of diesel engines. Using compacted graphite iron allows for thewall thicknesses of such blocks to be half those required by flake castiron. The compacted graphite iron block will have a weight of only about60 kilograms, but be stronger and stiffer than a flake cast iron block.

Compacted graphite irons exhibit a graphite shape intermediate betweenthat of stringy, interconnected flakes in gray iron and the dispersed,disconnected spheroids in ductile iron. As a result, the betterproperties of both gray and nodular iron are combined in compactedgraphite irons. The yield strength approaches that of ductile iron whilethe material retains the machining properties and castability of grayirons.

The chemistry of compacted graphite iron is essentially that of nodulariron except that, in processing, the nodularizing agent, such asmagnesium (or cerium), is either added in small proportions or isallowed to fade prior to casting. Alternatively titanium is added, sothat the graphite formation is changed to a compacted configuration asopposed to a spheroid. See Kovacs, et al. U.S. Pat. No. 4,596,606.

SUMMARY OF SELECT EXAMPLES OF EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a product comprising an ironcasting alloy comprising iron, niobium and compacted graphitestructures.

Another embodiment of the invention includes a product comprising aniron casting alloy comprising iron in an amount of about 88-91% byweight, carbon in an amount of about 3.0-3.6% by weight, silicon in anamount of about 4.0-4.60% by weight, niobium in an amount of about0.40-0.7% by weight, and wherein at least the portion of a carbon ispresent in the alloy as compacted graphite.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Select examples of embodiments of the invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 is an SEM illustrating graphite in gray iron.

FIG. 2 is an SEM illustrating graphite in ductile iron.

FIG. 3 is an SEM illustrating graphite in malleable iron.

FIG. 4 is an SEM illustrating graphite in compacted graphite ironaccording to one embodiment of the invention.

FIG. 5 is a graph of creep test data for a variety of alloy including aNiobium containing alloy according to one embodiment of the invention.

DETAILED DESCRIPTION OF SELECT EXAMPLES OF EMBODIMENTS

The following description of the embodiments is merely illustrative innature and are in no way intended to limit the invention, itsapplication, or uses.

One embodiment of the invention includes a product including an ironcasting alloy including iron, niobium and compacted graphite structures.

Another embodiment of the invention includes a product including an ironcasting alloy including iron (Fe) present in an amount of about 88-91%by weight, carbon (C) present in an amount of about 3.0-3.6% by weight,silicon (Si) present in an amount of about 4.0-4.6% by weight, niobium(Nb) present in an amount of about 0.40-0.70% by weight, and wherein atleast a portion of the carbon is present in the alloy as compactedgraphite structures.

Another embodiment of the invention includes a product including an ironcasting alloy including iron (Fe) present in an amount of about 88-91%by weight, carbon (C) present in an amount of about 3.0-3.6% by weight,silicon (Si) present in an amount of about 4.0-4.6% by weight, niobium(Nb) present in an amount of about 0.40-0.70% by weight, manganese (Mn)present in an amount 0.3% by weight or less, sulfur (S) present in anamount of about 0.02% by weight or less, phosphorus (P) present in anamount of about 0.07% by weight or less, nickel (Ni) present in anamount of about 0.6% by weight or less, titanium (Ti) present in anamount of 0.20% by weight or less, magnesium (Mg) present in an amountof about 0.05% by weight or less, and wherein at least a portion of thecarbon is present in the alloy as compacted graphite structures.Optionally, the alloy may also include molybdenum (Mo) as a substitutefor a portion of the niobium (Nb). However, due to cost niobium ispreferred. As such, another embodiment of the invention is free ofmolybdenum.

In one embodiment of the invention compacted graphite iron cast alloysmay be produced using graphite modifiers in the form of magnesium orcerium, the latter being made as additions in very small, regulatedamounts to the melt prior to solidification. When the magnesium orcerium content of the solidified structure is about 0.025%, nodulargraphite usually precipitates. Flake graphite is formed at magnesiumconcentrations below about 0.015% by weight. Thus, with magnesium orcerium concentrations in the range of 0.015-0.025% by weight, compactedgraphite (otherwise also referred to as vermiculite)) will precipitate.The addition of titanium to magnesium or cerium treated irons makes itpossible to produce compacted iron in both medium and heavy castings athigher magnesium and cerium concentrations. The presence of titaniumreduces the amount of control required on the magnesium concentrationand is considered beneficial in compacted graphite formation. Thus, witha magnesium addition containing titanium, compacted graphite, will formwith magnesium and/or cerium concentrations in the range of 0.015-0.04%by weight. In another embodiment, the magnesium and/or cerium is presentin a range of 0.015-0.035% by weight.

According to another embodiment of the invention a compacted graphiteiron cast alloy may be produced by a process including melting performedin a furnace heated to a temperature of about 2800-2850° F., and thenteamed into a ladle at a temperature of about 2750° F. Alloying elementsare added to the treating ladle along with graphite modifiers in theform of magnesium or cerium, with the optional addition of titanium.Commercial graphite modifying agents may include (a) rare earth elementsadded to a desulfurized iron, or (b) magnesium and titanium added priorto post-inoculation (slightly higher base sulfur can be used). Magnesiumor cerium may be used in an amount of about 0.15-0.04% by weight in acasting when titanium is used in an amount of about 0.08-15% by weight.The treated melt is then poured into one or more pouring ladles, and ateach of the pouring ladles a post-inoculant in the form of ferro-siliconor ferro-silicon with aluminum and calcium is added. The melt is thenpoured into molds at a temperature in the range of 2500-2600° F. and themold cooled without any special cooling treatment. Graphite modifyingagents may be added in a commercially available form which typically hasa composition of 52% silicon, 10% titanium, about 0.9% calcium, 5%magnesium, 0.25% cerium, with the modifier being added in an amount ofabout 0.5% of the total melt. The post-inoculant added to the pouringladle comprises ferro-silicon or titanium bearing ferro-silicon added inan amount of about 0.5% by weight. Copper may be added to the melt in anamount of about 0.4-1.9% by weight to maintain the carbon in the matrixof the casting microstructure. The casting may be thereafterheat-treated by any of a variety of methods known to those skilled inthe art.

In one embodiment of the invention at least 60% of graphite formationmay be present conforming to graphite type III with graphite size 5-8 inaccordance with ISO 945. The balance is conforming to graphite type VIwith the graphite size 5-8 in accordance with ISO 945. In oneembodiment, reticulate formation of vermicular graphite and chunkygraphite are prohibited. In one embodiment of the invention themicrostructure is free of cementit (Fe₃C). The overall matrixmicrostructure may include a minimum of 90% ferrite; the balanceconsists of pearlite and carbides.

A variety of tests are performed by those skilled in the art to providecritical design information regarding the strength of alloys, includingthose according to various embodiments of the invention. For example,the high temperature progressive deformation of a material at constantstress is called “creep.” in a creep test, a constant load is applied toa tensile specimen maintained at constant temperature, for example, atroom temperature. Strain is then measured over a period of time. Thedata collected is plotted in a curve of the strain rate or creep of thematerial. Stress rupture testing is similar to creep testing however,the stresses used are higher than the creep test and concludes when thematerial fans. FIG. 5 is a graph of creep test data for a variety ofalloy including data illustrate by line HSNb5.1 for a compacted graphitecast iron alloy wherein molybdenum has been replaced by niobiumaccording to one embodiment of the invention, Although the creep data iscomparable, with other alloy, alloys according to embodiment of theinvention have a higher thermal conductivity relative to ductile iron,and thus thermally induced stresses are much less. The elevatedtemperature strength of alloys according to embodiments of the inventionis less that a comparable grade ductile iron, but the thermalconductivity difference overcomes the deficit in strength.

Ductility of an alloy is used to indicate the extent to which the alloycan be deformed without fracture. One way of measuring the ductility isto determine the strain at which fracture occurs, which is usuallycalled the “elongation,” This measurement is obtained after fracture byputting the specimen back together and taking the elongationmeasurement. Because an appreciable fraction of the deformation will beconcentrated in a “neck” region of the tensile specimen, the value ofpercentage elongation will depend upon the length over which themeasurement is taken. If an alloy were prepared according to one of theembodiments of the invention described above, it is believed that itwould have a percent elongation of 10% or greater.

Hardness is also a measurement that characterizes an alloy. The HBWNumber expresses the hardness of an alloy as a ratio of the pressureapplied to a ball forced into the surface of the alloy to the surfacearea of the resulting indentation. If an alloy according to one of theembodiments of the invention described above were produced, it isbelieved that it would have a BHW Number ranging from 200-250 HBW.

One embodiment of the invention may include an iron casting alloyincluding iron, niobium and compacted graphite structures, and whereinthe alloy has the mechanical properties set forth in Table 1.

TABLE 1 Mechanical Properties At room temperature At 780° C.²⁾ Tensilestrength Rm [Mpa]¹⁾ Min. 550 Min. 70 Yield strength R_(p0,2)[Mpa]¹⁾ Min.470 Min. 60 (offset 0,2%) Elongation A [%] Min. 2 Min. 35 Hardness HBW10/3000 200-250 n.a. HBW 2,5/187,5 ¹⁾1Mpa = N/mm² ²⁾values onlyinformative

The mechanical properties (see Table 1) shall be verified in accordancewith EN 10002-1/ASTM E 8M with a round specimen of length L₀=50 mm (2Inches). The hardness shall be verified in accordance with ISO6506-1/ASTM E 10 (polished tungsten carbide ball with diameter D=10 mmand test force F=29.42 kN (3000 kgf) or polished tungsten carbide ballwith diameter D=2.5 mm and test force F=1.839 kN (187.5 kgf)).

The following description of select variants is only illustrative ofembodiments considered within the scope of the invention and is not inany way intended to limit such scope by what is specifically disclosedor not expressly set forth.

Embodiments may include Variant 1 which may include a productcomprising: an iron casting alloy comprising iron, niobium, andcompacted graphite structures.

Embodiments may include Variant 2 which may include a product as setforth in Variant 1 wherein the niobium is present in an amount rangingfrom 0.4-0.7% by weight of the alloy.

Embodiments may include Variant 3 which may include a product as setforth in one or more of Variants 1-2 wherein the alloy further comprisesmolybdenum.

Embodiments may include Variant 4 which may include a product as setforth in one or more of Variants 1-3 wherein the iron is present in anamount of about 88-91% by weight of the alloy.

Embodiments may include Variant 5 which may include a product as setforth in one or more of Variants 1-4 wherein the compacted graphitestructures comprise carbon present in an amount of 3.0-3.6% by weight.

Embodiments may include Variant 6 which may include a product as setforth in one or more of Variants 1-5 further comprising silicon presentin an amount of 4.0-4.6% by weight of the alloy.

Embodiments may include Variant 7 which may include a product as setforth in one or more of Variants 1-6 further comprising manganesepresent in an amount up to 0.3% by weight.

Embodiments may include Variant 8 which may include a product as setforth in one or more of Variants 1-7 further comprising sulfur presentin an amount up to 0.02% by weight of the alloy.

Embodiments may include Variant 9 which may include a product as setforth in one or more of Variants 1-8 further comprising phosphoruspresent in an amount up to 0.07% by weight of the alloy.

Embodiments may include Variant 10 which may include a product as setforth in one or more of Variants 1-9 further comprising nickel presentin an amount up to 0.6% by weight of the alloy.

Embodiments may include Variant 11 which may include a product as setforth in one or more of Variants 1-10 further comprising titaniumpresent in an amount up to 0.2% by weight of the alloy.

Embodiments may include Variant 12 which may include a product as setforth in one or more of Variants 1-11 further comprising a graphitemodifier present in an amount ranging from 0.015-0.04 weight percent ofthe alloy, the graphite modifier comprising at least one of magnesium orcerium.

Embodiments may include Variant 13 which may include a product as setforth in one or more of Variants 1-12 further comprising titanium.

Embodiments may include Variant 14 which may include a product as setforth in one or more of Variants 1-13 wherein the graphite modifier ispresent in an amount ranging from 0.015-0.025 weight percent of thealloy.

Embodiments may include Variant 15 which may include a product as setforth in one or more of Variants 1-14 wherein the alloy has amicrostructure that is free of Fe₃C.

Embodiments may include Variant 16 which may include a product as setforth in one or more of Variants 1-15 wherein at least 60 percent of thegraphite structures conform to graphite type III with graphite size 5-8according to the standard ISO 945.

Embodiments may include Variant 17 which may include a product as setforth in one or more of Variants 1-16 wherein balance of the graphitestructures conform to graphite type IV with graphite size 5-8 accordingto the standard ISO 945.

Embodiments may include Variant 18 which may include a productcomprising an iron casting alloy comprising iron present in an amount ofabout 88-91% by weight, carbon present in an amount of about 3.0-3.6% byweight, silicon present in an amount of about 4.0-4.6% by weight,niobium present in an amount of about 0.4-0.7% by weight, manganese inan amount of 0.05% or less by weight, sulfur in an amount of about 0.02%by weight or less, phosphorus in an amount of about 0.07% by weight orless, nickel in an amount of about 0.6% by weight or less, wherein thecarbon is present in a form comprising compacted graphite structures.

Embodiments may include Variant 19 which may include a product as setforth in Variant 18 further comprising titanium in an amount rangingfrom about 0.08-0.2% by weight of the alloy.

Embodiments may include Variant 20 which may include a product as setforth in one or more of Variants 18-19 wherein at least 60 percent ofthe graphite structures conform to graphite type III with graphite size5-8 according to the standard ISO 945.

Embodiments may include Variant 21 which may include a product as setforth in one or more of Variants 18-20 wherein balance of the graphitestructures conform to graphite type IV with graphite size 5-8 accordingto the standard ISO 945.

Embodiment may include Variant 22 which may include a product as setforth in one or more of Variants 18-21 wherein the alloy has amicrostructure free of Fe₃C.

Embodiments may include Variant 23 which may include a product as setforth in one or more of Variants 18-22 further comprising molybdenum.

Embodiments may include Variant 24 which may include a method of makingcompacted graphite iron, comprising: forming a ferrous alloy meltcomprising iron present in an amount of about 88-91% by weight, carbonpresent in an amount of about 3.0-3.6% by weight, silicon present in anamount of about 4.0-4.6% by weight, niobium present in an amount ofabout 0.4-0.7% by weight, and causing the melt to form compactedgraphite particles upon solidification; solidifying said melt to form acompacted graphite iron casting.

Embodiments may include Variant 25 which may include a method as setforth in Variant 24 wherein the forming comprises heating the elementsto a temperature of 2800-2850° F. prior to the solidifying.

Embodiments may include Variant 26 which may include a method as setforth in one or more of Variants 24-15 wherein the melt furthercomprises a graphite modifying agent is present in an amount rangingfrom 0.015-0.035% by weight, and wherein the graphite modifying agentcomprises at least one of magnesium and cerium.

Embodiments may include Variant 27 which may include a method as setforth in one or more of Variants 24-26 wherein the melt is free ofmolybdenum.

Embodiments may include Variant 28 which may include a product as setforth in one or more of Variants 14-27 wherein the casting is free ofFe₃C.

Embodiments may include Variant 29 which may include a product as setforth in one or more of Variants 24-28 wherein at least 60 percent ofthe graphite structures conform to graphite type III with graphite size5-8 according to the standard ISO 945.

Embodiments may include Variant 30 which may include a product as setforth in one or more of Variants 24-29 wherein balance of the graphitestructures conform to graphite type IV with graphite size 5-8 accordingto the standard ISO 945.

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

1. A product comprising: an iron casting alloy comprising iron, niobium,and compacted graphite structures.
 2. A product as set forth in claim 1wherein the niobium is present in an amount ranging from 0.4-0.7% byweight of the alloy.
 3. A product as set forth in claim 2 wherein thealloy further comprises molybdenum.
 4. A product as set forth in claim 1wherein the iron is present in an amount of about 88-91% by weight ofthe alloy.
 5. A product as set forth in claim 1 wherein the compactedgraphite structures comprise carbon present in an amount of 3.0-3.6% byweight.
 6. A product as set forth in claim 5 further comprising siliconpresent in an amount of 4.0-4.6% by weight of the alloy.
 7. A product asset forth in claim 5 further comprising manganese present in an amountup to 0.3% by weight.
 8. A product as set forth in claim 5 furthercomprising sulfur present in an amount up to 0.02% by weight of thealloy.
 9. A product as set forth in claim 5 further comprisingphosphorus present in an amount up to 0.07% by weight of the alloy. 10.A product as set forth in claim 5 further comprising nickel present inan amount up to 0.6% by weight of the alloy.
 11. A product as set forthin claim 5 further comprising titanium present in an amount up to 0.2%by weight of the alloy.
 12. A product as set forth in claim 1 furthercomprising a graphite modifier present in an amount ranging from0.015-0.04 weight percent of the alloy, the graphite modifier comprisingat least one of magnesium or cerium.
 13. A product as set forth in claim12 further comprising titanium.
 14. A product comprising an iron castingalloy comprising iron present in an amount of about 88-91% by weight,carbon present in an amount of about 3.0-3.6% by weight, silicon presentin an amount of about 4.0-4.6% by weight, niobium present in an amountof about 0.4-0.7% by weight, manganese in an amount of 0.05% or less byweight, sulfur in an amount of about 0.02% by weight or less, phosphorusin an amount of about 0.07% by weight or less, nickel in an amount ofabout 0.6% by weight or less, wherein the carbon is present in a formcomprising compacted graphite structures.
 15. A method of makingcompacted graphite iron, comprising: forming a ferrous alloy meltcomprising iron present in an amount of about 88-91% by weight, carbonpresent in an amount of about 3.0-3.6% by weight, silicon present in anamount of about 4.0-4.6% by weight, niobium present in an amount ofabout 0.4-0.7% by weight, and causing the melt to form compactedgraphite particles upon solidification; solidifying said melt to form acompacted graphite iron casting.